Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. By reducing displacement under low torque request situations, the engine is operated at a higher manifold pressure, reducing engine friction due to pumping, and resulting in reduced fuel consumption.
There are a few examples of how deactivating engine cylinders is typically conducted in a four-stroke engine that includes intake, compression, combustion (power), and exhaust strokes. In a first example, during the intake stroke an air-fuel charge is drawn into the cylinder, the air-fuel charge is compressed, spark is provided resulting in combustion, but rather than exhausting the combustion gases, the exhaust valve is maintained closed. This traps the high-pressure charge in the cylinder. An advantage to this methodology is lower oil migration/consumption as the high pressure in the deactivated cylinder prevents migration of oil into the cylinder. However, such a method has a distinct disadvantage in that there is a noticeable torque bump at deactivation, and a pumping penalty is realized for the first event after deactivation (which is small if deactivating for short periods, but significant if deactivated and reactivated often).
Another example of cylinder deactivation includes the same steps as above in the first example, but rather than trapping the high-pressure charge, the cylinder is exhausted, but rather than re-inducting an intake charge after the exhaust stroke, a vacuum is trapped in the cylinder by closing the exhaust valve (while maintaining closed the intake valve). Such an example has an advantage over the first example, in that there is a reduction in noticeable torque bump at deactivation, and increased fuel efficiency when deactivated and activated often, due to lower pumping work. However, a distinct disadvantage to such methodology is that by trapping the vacuum in the cylinder, oil consumption may increase. Increased oil consumption may result in at least two undesirable issues. The first may include oil fouled spark plug(s). A second issue may include the fact that crankcase vapors and/or oil migrating from a crankcase to the cylinder may result in a combustible mixture, which may result in a combustion event in the cylinder. Disabling spark during cylinder deactivation may prevent unintended combustion of crankcase vapors/oil migration, however oil fouling and oil migration may still result in spark plug degradation (spark plug fouling).
U.S. Pat. No. 9,261,067 B2 teaches a method for reducing spark plug fouling in deactivated cylinder(s), comprising supplying spark at a particular determined instance while the cylinder(s) are deactivated. However, the inventors have recognized an issue with such an approach. For example, the supplying of spark is not specified with relation to engine cycle status, or position of a piston(s) coupled to the cylinder(s). As such, providing spark according to U.S. Pat. No. 9,261,067 B2 may result in undesired combustion events while the cylinder(s) is deactivated.
Furthermore, in the event of unintended combustion, even if mitigating actions are taken to avoid such unintended combustion, a noticeable torque bump may result and residual combustion gases in the cylinder that was the source of the unintended combustion may result in inefficient combustion upon reactivation of the particular cylinder. The inventors have herein recognized these issues, and have developed systems and methods to at least partially address them. In one example, a method comprises deactivating a subset of cylinders of a variable displacement engine while other cylinders of the engine combust air and fuel, reducing or avoiding a torque bump due to an unintended combustion event in a deactivated cylinder by reducing a torque output of the engine and reactivating the deactivated cylinder which had the unintended combustion event, and during subsequent cylinder deactivation events, not deactivating the cylinder which had the unintended combustion event. In this way, under conditions where an unintended combustion event occurs while operating the engine with deactivated cylinders, a torque bump associated with the unintended combustion event may be reduced or avoided, which may improve engine efficiency and which may improve customer satisfaction.
In one example of the method, the unintended combustion event includes an acceleration of a crankshaft coupled to the engine greater than a threshold crankshaft acceleration. Furthermore, in some examples, the method may include exhausting residual burnt gas from the deactivated cylinder with the unintended combustion event prior to reactivating the deactivated cylinder with the unintended combustion event.
In another example of the method, reducing the torque output of the engine may comprise reducing a torque contribution of an activated cylinder that is scheduled to combust air and fuel immediately following the unintended combustion event. In such an example, reducing the torque contribution of the activated cylinder may further comprise retarding a spark provided to the activated cylinder for combustion of air and fuel, where an amount that the spark is retarded is a function of a torque increase provided to the engine via the unintended combustion event.
Still further, in an example of the method, the method may further comprise deactivating the subset of cylinders of the engine via trapping either a negative pressure with respect to atmospheric pressure or a positive pressure with respect to atmospheric pressure in the subset of cylinders. As one example, trapping the negative pressure may be in response to an indication that an oil quality of an oil utilized for cooling, lubrication and/or cleaning of the engine is greater than an oil quality threshold, and wherein trapping the positive pressure may be in response to an indication that the oil quality of the oil is below the oil quality threshold. Furthermore, such a method may comprise providing spark to the subset of deactivated cylinders at a predetermined position of one or more pistons coupled to the subset of deactivated cylinders, wherein providing spark may further be a function of pressure in the subset of deactivated cylinders, and wherein providing spark may serve to prevent fouling of one or more spark plugs configured to provide spark to the subset of deactivated cylinders. In one example, the predetermined position of the one or more pistons may include a position within a threshold number of degrees from a bottom dead center position.
Finally, in an example of the method, the engine may comprise a variable displacement engine. In such an example, reactivating the deactivated cylinder may further comprise reactivating the subset of deactivated cylinders including the deactivated cylinder which had the unintended combustion event, and deactivating the other cylinders of the engine operating to combust fuel.
In this way, spark plug fouling may be prevented in deactivated engine cylinders, and in the event that unintended combustion occurs in one of the deactivated engine cylinders, mitigating action may be undertaken to avoid a resulting torque bump.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.